1. Field of the Invention
The present invention relates to volatile memory backup systems including all-solid-state batteries.
2. Description of the Related Art
Volatile memories, such as DRAMs, are used as main memories of computers and servers. Especially DRAMs, which provide significantly high processing speed, have recently been used widely in computers and servers. At the same time, volatile memories, such as DRAMs, have such characteristics as to lose stored data when power supply is interrupted; thus, uninterruptible power systems (UPSs) or power generators (such as diesel power generators) are typically installed together with the volatile memories, for example, in the servers in mission-critical systems in preparation for power failures such as a power outage and an instantaneous voltage drop. Such precautionary measures facilitate smooth system restoration in the event of power failures. Unfortunately, since these devices are designed to supply power to the entire devices, such as servers, they should have a large scale and are typically installed separately from the devices, such as servers. Hard disk drives may be used as data storage for DRAMs; however, they take time (for example, several hours) to restore data to the DRAMs after the restoration of power supply.
Another type of DRAM is commercially available that includes a supercapacitor functioning as a small backup power source. Examples of the DRAMs provided with such supercapacitors are ArxCis-NV™ from Viking Technology and an NVDIMM from Micron Technology, Inc. Such a DRAM with a supercapacitor can transfer data stored in the DRAM to a nonvolatile memory (such as a NAND flash memory) using power temporarily supplied from the supercapacitor in the event of power failures such as a power outage and an instantaneous voltage drop and thus can retain the stored data in the nonvolatile memory after the stop of the power supply from the capacitor. After the power failure ends and then the power supply restarts, the stored data in the nonvolatile memory is restored into the DRAM, which enables prompt system restoration. Unfortunately, a supercapacitor (even if small) is too large to be mounted in a memory module, and has low heat resistance. The data in the DRAM is lost at the stop of power supply even within one minute, and thus it takes time (for example, several tens of seconds) to restore the data from the flash memory to the DRAM after the restoration of power supply.
A battery-backup DRAM has also been proposed that is a combination of a DRAM and a battery. At the stop of power supply, the battery-backup DRAM retains the data in the DRAM for a certain time by (1) switching the DRAM to a power-saving mode (for example, a self-refresh mode) specializing in data retention and (2) activating an emergency power source (for example, a lithium secondary battery) for retaining data in the DRAM. Unfortunately, the battery does not offer resistance to high temperature and has low energy density, so that the battery is difficult to be mounted in a memory module, and leads to loss of the data in the DRAM at the stop of power supply for a time over a designed back-up time.
The recent development of portable devices, such as personal computers and cellular phones, has been greatly expanding the demand for batteries as the power sources of the devices. Traditional batteries for such application contain liquid electrolytes (electrolytic solutions) containing flammable organic diluent solvents, as media for ion transfer. The battery containing such an electrolytic solution has a risk of the leakage of the electrolytic solution, ignition, explosion, or the like. To solve the problems, an all-solid-state battery has been developed that contains a solid electrolyte instead of a liquid electrolyte and consists of only solid components for ensuring the intrinsic safety. The all-solid-state battery, which contains a solid electrolyte, has a low risk of ignition, causes no liquid leakage, and barely causes a decline in the battery performance due to corrosion. The recent expansion of the application of batteries has been demanding smaller batteries with larger capacity. For example, such batteries include an all-solid-state battery that has a thick positive electrode and increased capacity. Patent Document 1 (U.S. Pat. No. 8,431,264) and Patent Document 2 (JP2009-516359A) each disclose an all-solid-state battery including a positive electrode having a thickness of more than approximately 4 μm and less than approximately 200 μm, a solid electrolyte having a thickness of less than approximately 10 μm, and a negative electrode having a thickness of less than approximately 30 μm. The positive electrodes disclosed in these documents apparently are composed of non-oriented positive-electrode active materials.
An oriented sintered plate made of lithium complex oxide has been proposed. For example, Patent Document 3 (JP2012-009193A) and Patent Document 4 (JP2012-009194A) each disclose a lithium-complex-oxide sintered plate having a layered rock-salt structure and having a diffraction intensity ratio [003]/[104] of 2 or less of the (003) plane to of the (104) plane in X-ray diffraction. Patent Document 5 (JP4745463B) discloses platy particles that are expressed by the general formula: Lip(Nix,Coy,Alz,)O2 (where 0.9≦p≦1.3, 0.6<×<0.9, 0.1<y<0.3, 0≦z≦0.2, and x+y+z=1), that have a layered rock-salt structure, and in which the (003) plane is oriented so as to intersect the plate surface of the particle.
A garnet-type ceramic material having a Li7La3Zr2O12-based (LLZ-based) composition has received attention as a solid electrolyte having lithium-ion conductivity. For example, according to Patent Document 6 (JP2011-051800A), addition of Al to Li, La, and Zr, which are the basic elements of LLZ, improves denseness and lithium-ion conductivity.
According to Patent Document 7 (JP2011-073962A), addition of Nb and/or Ta to Li, La, and Zr, which are the basic elements of LLZ, further improves lithium-ion conductivity. Patent Document 8 (JP2011-073963A) discloses that a composition containing Li, La, Zr, and Al in a molar ratio of Li to La of 2.0 to 2.5 further improves denseness.